String interpolation shows up as "path segment" in trace

[roberth]

Describe the bug

See tests/functional/lang/eval-fail-toString-returns-non-coercible.err.exp (exposed in #16158)

This trace item should report "string interpolation" or some such:

… while evaluating a path segment
         at /pwd/lang/eval-fail-toString-returns-non-coercible.nix:2:2:
            1| # interpolation checks the return type of __toString
            2| "${{ __toString = self: null; }}"
             |  ^
            3|

Steps To Reproduce

see above

Expected behavior

see above

Metadata

Additional context

• Path interpolation reuses AST types too cleverly, subjectively
• Exposed in lazy paths prep: Peel and coerce #16158

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates
• reviewed the contributing guide

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